Hooray! It's Cost of Government Day. Or maybe there isn't cause for cheer. Because unlike its flashier, happier sister Tax Freedom Day, Cost of Government Day isn't the first day of anything, but the last day of something. It's the day when government spending gives way to private spending. Let me explain.

Imagine that we spent an equal amount of our net national income each day: one three-hundred-and-sixty-fifth of the total every 24 hours. Imagine further that instead of splitting spending among all its different targets as and when you need the goods and services you're buying, the government did all of the spending until it had finished. Then, afterward, all spending is private spending. Friday June 24th—today—is the last day where spending is done by the government and from tomorrow—Saturday June 25th—all spending is private.

Let me caveat that claim a bit. Firstly, just to be clear, it's obvious that government spending does not proceed in this way, it's spread across the year according to programme needs and so on. Secondly, we're looking at projected numbers, and we've used financial year figures to calculate calendar years. Thirdly, we've compared government spending to net national income, not gross domestic product, and GDP is much closer to a measure of total spending (indeed, it is partly calculated from a measure of expenditures).

But all of these caveats come with good reasons. Presenting government spending as a single chunk gives people a clearer and much more tangible grasp on its size. Most people don't think in financial or tax years, and when we revise our figures in future years, the changes are mostly small, indicating that our estimates usually aren't far off.

We even think there's a good reason for using net national income. One reason we track Cost of Government Day is to give an indication of the sheer scale of the UK government. But another is to give an indication of where future Tax Freedom Days could end up—and net national income is more close to a measure of the incomes we'll use to bear that burden.

When there is sufficient demand in the economy, government borrowing, just as much as taxation, comes at the expense of private economic activity. Money that is lent to the government cannot be invested in other projects.

What's more, presuming we don't want to throw away centuries of governmental financial probity, or create very high and unexpected inflation, debts must eventually be paid. Yes, we can grow the debt away: if the economy expands at a higher rate than the interest on our debt, the burden will get ever smaller relative to the amount we are producing each year. But £10bn of extra debt is still £10bn more (plus interest) in taxes to raise at some point.

Tax Freedom Day is more immediately appreciable, but arguably, Cost of Government Day is even more important. Government projects are often nice, but they inevitably come with a cost down the line. This year there was around a month between TFD & COGD—a month of pure borrowing—and one day that will be your loan to pay back.